The present invention relates to watercraft and pertains particularly to an improved fin for surfboards and other small watercraft.
Many watercraft, particularly surfboards, windsurf boards, and sailboats use a fin to resist side slip as a result of transverse forces on the craft. The transverse forces may be applied to the watercraft from many sources such as wind, traversing the slope of a wave and executing a turn. When a surfboard is traversing a wave, the fin extends into the water and helps resist and/or prevent side slip of the board on the face of the wave. Because the board is at an angle to the surface of the wave, the fin frequently has very little of its length in the water. The result is that the net effective area of the fin is decreased.
When a conventional fixed fin surfboard is turning, as illustrated in FIG. 1, the rail toward the turn (left) dips down and the board 12 is tilted resulting in the fin 14 extending at an angle to the water surface and presenting less area to resist lateral forces. This results in the fin becoming less effective in resisting the lateral forces. The effective area of the fin can be expressed by the formula n=d sine .phi.. Where n is a major component of the area of the fin, d is depth or length of the fin and .phi. is the angle of the fin. Thus, the effective area component n of the fin is decreased when the board is in a turn making it less effective in the turn. The force F on the fin is directly proportional to the effective fin area and is therefore reduced when in a turn. The same thing occurs when a sailboard or sailboat keels over under the force of the wind.
It is therefore desirable to have a fin that is more effective in a turn or heel condition of the craft.